The invention relates to a circuit board and a method of manufacturing a circuit board.
Circuit boards are currently manufactured by laminating a plurality of two-sided photolithographically structured individual layers precisely onto each other. The starting material for the single layers is a reinforced or non-reinforced film or sheet of epoxy resin material which in general is already provided with a thin copper layer as a semi-finished product in a nonstructured form. Since the contact vias are produced after laminating by making bore holes, the individual layers have to be layered very precisely on top of each other, for a drilling tool to reliably hit the contacts to be connected. As an alternative, bore holes which are produced with the required high precision and in correct alignment in relation to the conductor tracks may also be used as positioning formations. However, since this is necessary for each individual layer with very high precision and, hence, great expenditure, the manufacturing of circuit boards is expensive.
In this technology, components may be mounted only on the upper side or on the underside of the multi-layer circuit board. The components may be active semiconductor components provided with a package or in the form of a chip, or passive components. Such components are not suitable for being mounted on the inner layers of the circuit board since it is not possible to provide space between the layers, such as in the form of an accommodation opening. There is therefore an increase in the wiring expenditure, and intermediate layers additionally required make the circuit board more expensive and result in an increase in its complexity and size.
In modern circuit boards a radio-frequency line is realized on a layer level by two electric single conductors extending in parallel. The electric field is necessarily led between the two conductors in the circuit board material. Because of its dielectric material properties, the circuit board material is thereby given a substantial influence on the transmission properties of the RF line. As frequencies increase, standard materials such as FR4 are no longer suitable, which is why ceramic substrates or fluorinated plastics (Teflon) are utilized, which are then suitable up to higher frequencies. But these materials are expensive and have likewise a maximum frequency beyond which the RF line, due to its material, is damped too strongly.
In conventional circuit boards it would also be possible to integrate optical intermediate layers with dielectric waveguides. However, for coupling the light in and out, a precise positioning has to be performed between the different layers. This is not feasible since the starting material is plate-shaped and non-structured and the structuring of the copper foil is not suitable for positioning semiconductor chips and plates in relation to each other.
For dissipating large amounts of lost heat the known circuit boards could also be provided with an integrated fluidic cooling system. The cooling channels, however, need to be manufactured in separate operations, which makes the circuit board expensive.
With the increase in the clock rate of the information processing systems in which the prior circuit boards are employed there is also an increase in the demands made on the compactness of the circuit board, the transmission frequency of the RF conductors, the thermal power loss to be dissipated, and the precision of the electric conductors. In addition, a need will arise to integrate optical lightguides into the circuit board, it being required to be possible to couple light in and out at low expenditure. It is a disadvantage of the prior known circuit boards that an improvement in the RF behavior, an integration of optical waveguides and fluidic cooling systems as well as an increase in the packing density is possible only at an extremely high expenditure.
The object of the present invention consists in providing a circuit board which creates the prerequisites for a high packing density, good RF conduction, a cooling system adapted to be designed in a simple manner, and the use of optical waveguides.
This object is achieved by a circuit board consisting of at least two individual circuit board layers made of plastics and produced by formation technique, which each have first and second functional sides and at least one microstructured positioning formation on the first and second functional sides and at least one microstructured conductor trench on one of the functional sides, the conductor trench being provided with a metallization. By use of a formation of molding process, in particular an injection molding process, for producing the individual layers, the formations required for the desired functions of the circuit board may be configured at low expense and with maximum precision already during the manufacture of the individual layers; there is no need for a subsequent machining step such as a finishing cutting operation, in order to configure geometric structures with a high positional accuracy. Since the positioning formations are also formed already during the production of the individual layers, the arrangement thereof in relation to microstructures arranged on the functional sides and having specific functions is predefined precisely, so that all of the microstructures of the individual layer will then later be in precise alignment with respect to each other.
According to one embodiment of the invention, protrusions and depressions which are pyramid-shaped, for example, may be used for the positioning formations. When the individual layers are placed on top of each other to form the circuit board, the protrusions of the one individual layer engage into the depressions of the other individual layer, so that an automatic alignment of the individual layers results in relation to each other.
In accordance with an alternative embodiment of the invention the positioning formations may be in the form of openings which extend from the one functional side through the individual layer and as far as to the other functional side. A positioning pin may then be passed through these openings, so that the individual layers are precisely positioned in relation to each other.
According to one embodiment of the invention, provision is made that the conductor trench extends as far as to the edge of the circuit board, so that a plug connector may be connected. This plug connector may either be slipped onto the entire circuit board or, if this turns out to be expedient, only onto some of the individual layers, which are provided for this purpose with a connection protrusion projecting from the circuit board; the respective conductor trenches then extend as far as onto the connection protrusion.
For obtaining an RF line, in accordance with one embodiment of the invention a first conductor trench is provided on one of the individual layers and a second conductor trench is provided on the other individual layer, the two conductor trenches being located centered opposite each other and one of the conductor trenches having smaller dimensions than the other conductor trench. The conductor trench having the larger dimensions may be semicircular or rectangular in cross-section, for example, so that it bridges over the conductor trench having the smaller dimensions, the space between the conductor trenches located opposite each other being filled with air. In this way an RF conductor is formed which is arranged xe2x80x9cbetweenxe2x80x9d the individual layers of the circuit board. The electric and magnetic alternating field is guided in the enclosed cavity of air, so that the material properties of the plastics of the individual layers have no influence on the RF line.
For cooling the circuit board, in accordance with one embodiment of the invention a cooling groove is provided on at least one of the individual layers, the cooling groove being filled with a metallization of a thickness such that a heat sink is formed. Such heat sink allows dissipating the thermal loss from components mounted on the circuit board by thermal conduction.
For cooling the circuit board, in accordance with another embodiment of the invention a cooling channel may be provided on at least one of the individual layers, the cooling channel being adapted for a cooling agent to be conducted therethrough, the other individual layer covering the cooling channel. The cooling channel, too, presents a microstructure which can be designed without much expenditure during formation of the individual layer. By means of the cooling channel an active cooling of the circuit board can be achieved, whereby even very large thermal losses may be dissipated.
The cooling channel preferably extends as far as to the edge of the circuit board, a connection for the cooling agent being formed on the circuit board. By means of the connection for the cooling agent the cooling channel may be connected in a simple manner to an external cooling device which ensures the cooling agent supply.
According to a further preferred embodiment of the invention, at least one mount for an electronic, optical or optoelectronic component is provided in at least one of the individual layers. The mount likewise presents a microstructure which may be simply formed with high accuracy during production, so that the components may be arranged at exactly the right place in the interior of the circuit board, which reduces the expenditure for connecting the components. If required, a recess located opposite the mount may be provided in the other individual layer. This recess ensures, for example, the prevention of damage to the bonding wires used for connecting the component.
The individual layers of the circuit board may be connected with each other by an electrically conductive material such as, e.g., by an electrically conductive adhesive. This allows a contacting of conductor trenches which are arranged on different circuit boards. For the contacting of conductor trenches which are arranged on functional sides, facing away from each other, of the individual layers, contact openings may be used which extend from the first functional side of an individual layer through the layer and as far as to the second functional side and are filled with an electrically conductive material.
In accordance with a further preferred embodiment of the invention, provision is made that at least one of the individual layers consists of an optically transparent material and that on this individual layer a waveguide trench is provided which is filled with an optically transparent material the refractive index of which suitably differs from that of the material of the individual layer, so that a waveguide is formed. The microstructure of the individual layer required for the manufacture of the waveguide may also be produced at low expense during formation of the individual layers. Thereafter, it is merely necessary to introduce a suitable material into the waveguide trench. Optoelectronic components adapted for being suitably arranged in the mounts of the individual layers can then cooperate with the waveguide. Owing to the precise arrangement of the mounts in relation to the waveguide, it is possible to reliably achieve good optical coupling between the components arranged in the mounts and the waveguide.
Preferably, provision is made that the individual layer provided with the waveguide comprises a mirror by means of which light may be coupled into and out of the waveguide. This allows the use of an optoelectronic component which radiates light in a direction perpendicular to the plane in which the waveguide extends.
It is preferably provided that the mirror is a separate component which is inserted in the individual layer. This reduces production costs since the mirror and the individual layer may be produced independently from each other.
The above-mentioned object is further achieved by a method of manufacturing a circuit board, comprising the following steps: first at least two individual layer blanks are produced by formation from a casting of molding process, each of the blanks being provided with positioning formation preforms on first and second functional sides. The individual layer blanks are then subjected to a pretreatment on their entire surface such that they can be provided with a metallization. The pretreatment may consist, for example, in that a thin pre-metallization is applied or the substrate is seed-injected. In those regions which are not intended to be provided with a metallization, the surface is subjected to a subsequent treatment, so that no metallization is deposited in these regions. The subsequent treatment may consist, for example, in that the pre-metallization is taken away mechanically or the seeding is removed chemically. Subsequently, a metallization is applied to the regions which have not been subjected to a subsequent treatment. It is in this way that the conducting tracks are formed on the printed circuit board. Finally, the individual layer blanks are placed on top of one another and at the same time precisely positioned in relation to each other by means of the positioning formations. The basic principle is to spend the high expenditure which is required for achieving the required precision only one time, namely on producing the mold from which the individual layers are formed. If this mold has been produced with the required accuracy, the desired geometric microstructures such as the positioning formations, the mounts for components as well as the cooling channels may be formed without any further large expenditure.
Advantageous designs of the invention will be apparent from the subclaims.